The "BF" is more likely a "BP". It is not a mintmark, it is the initials of the designer of the most commonly used "St. George and the Dragon" design, Benedetto Pistrucci.
The Pistrucci design appears most commonly on the British Five Pound, Two Pound, Sovereign and Half-Sovereign gold coins since 1818. It has also appeared regularly on predecimal Crown (Five Shilling) coins since 1818.
It's NOT a mintmark. BLP are the initials of Bela Lyon Pratt who designed this coin.
A coin of 1913 cant be of Queen Victoria as she died in 1901, the veiled head was used from 1893 to her death in 1901. A coin showing St George and the dragon is 5 Shillings (A Crown) if Silver, and is either a sovereign or half sovereign (20 or 10 Shillings) if gold. The BM mark is the designer of the Monarchs head on British coins, But was only used from 1911 onwards on the coins of George V. BM stands for Bertram MacKennal. This Coin cannot be a legal tender issue, but could be some type of gaming token, as stated, Queen Victoria had died 9 years before BM showed on coins, but would be inline with a coin of 1913 but depicting king George V. A Crown of 1913 was not issued that year, so would be one of the smaller gold coins.
The mint mark on a $5 gold coin from that era is on the reverse (tails) side, just below the eagle.
In EF-40 retail is $300.00, in EF-45 $325.00 The value is approximately $255.00, this does not include any sellers mark up.
You need to provide a denomination, an estimate of condition and a mint-mark (if any) for us to accurately evaluate your coin.
About $1000 for its gold content, regardless of mint mark.
If the coin has one, the mintmarks are on the reverse under the eagle.
The value of a 20 mark gold coin from 1909 would depend on its condition, rarity, and current market demand. You would need to consult with a numismatic expert or appraiser to get an accurate valuation for this specific coin.
For coins dated 1906 to the present, it means the coin was made at the Denver Mint. A "D" mint mark on an old (1838-1861) gold coin means it was made at the long-closed gold coin mint at Dahlonega, GA.
It is only gold plated. This adds no value to the coin. It is considered an altered coin and is worth 25 cents.
Assuming no mint mark and the coin is circulated, retail values are $330.00-$500.00
$450 to $500 regardless of mint mark, due to its gold content.